We must look to the solar system for examples of stars in the last stage of development. Each of the planets may in fact be regarded as an object of this kind. The bare and rocky surface of the moon affords a desolate picture of what may result from this long continued process of condensation. The volcanic region which is shown to excellent advantage in a photograph recently taken with the Yerkes telescope, (Fig. 13) gives no evidence of the existence of life; in fact, the spectroscope indicates that if there is any air on the moon it is much too rare to support life as we know it.
Fortunately, the moon is not the only example of a worn-out star. The earth, which probably has many counterparts in the universe, is another example of a less desolate kind. Here, though the process of condensation which is the chief cause of celestial phenomena has ceased, the problem of evolution has not ended. In fact, though the cosmical problems which we have considered in their barest elements will not be completely solved for centuries, it may be truly said that the questions raised by the countless living organisms in a single drop of ditch water are still more complex, and will require a still longer time for their solution.